The present invention relates to improved methods of fabricating ultrasonic sensor arrays used to form ultrasonic images. Such sensors are used in applications such as ultrasonic, non-invasive medical imaging. The invention is particularly directed to methods of fabricating hermetically sealed sensor arrays. Arrays produced using this method will have superior acoustic performance because their impedance matching can be optimized.
An ultrasonic array works the same way a sonar system does. The major difference is that the distance from the ultrasonic array to the target is much shorter than the distance from a sonar to its target. During the transmit phase, the transducer array acts as a generator of ultrasonic energy. During the listening, or receiving, phase the transducer array acts as a sensor of reflected ultrasonic energy. In both cases, the ultrasonic array elements act as transducers. During transmission, they convert electrical energy into ultrasonic energy; during reception, they convert ultrasonic energy into electrical energy.
The ultrasonic beam is pointed in a particular direction during this transmit-receive sequence, and ultrasonic energy is received from different distances into the target in the given direction; the amount of energy received corresponds to the amount of acoustic energy reflected within the target. An ultrasonic "image" is formed by sequentially pointing the array in different directions, so that an image is built up from a large number of individual point images. Usually, the sensor is physically scanned back and forth in two directions, thereby performing a "2-sector scan" usually at a rate of about 10 Hertz, corresponding to twenty sector scans per second.
An ultrasonic point image of an object or target, such as an organ within the human body, is formed by sending out one or more pulses of ultrasonic energy from an ultrasonic array, so that the pulses are coupled into the object. The ultrasonic array then "listens" for echoes from within the object. Echoes occur at any location where there is a change in the object's acoustic properties. A change occurs wherever the velocity of sound changes. Such a change in sound velocity is referred to as a change in "acoustical impedance". The acoustic impedance changes, for example, at the interface between blood and soft tissue. Acoustical impedance changes are necessary if ultrasonic imaging is to occur, because without acoustical impedance changes there would be no change in reflected energy and hence no image formed.
However, large acoustical impedance mismatches close to the ultrasonic array are undesirable. Acoustical impedance mismatches at the transmitter or receiver reduce the amount of energy transmitted into the "target" or received back from the target. Without "impedance matching" the sensor array to the object, only a small fraction of the ultrasonic energy generated will pass into the target. Similarly, without impedance matching, only a small fraction of the energy returned from the target will be received by the sensor array.
Thus, in order to efficiently couple ultrasonic energy into the object being imaged, such as the human body, the impedance of the array and the object must be closely matched. Impedance matching requires that the velocity of the acoustic energy undergo a gradual change, rather than an abrupt change. The impedance matching is done by means of special coatings placed on the sensor array.
For example, in order to facilitate impedance matching between the ultrasonic array and the human body, the transducer is mounted inside a flexible liquid filled container with an acoustic window, and the window is placed against the body. The liquid and the flexible container provide a good impedance match to the human body, while the array can be mechanically scanned inside the liquid. The array is impedance matched to the liquid in the container by one or more layers of impedance matching material bonded to the concave face of the array.
In order to focus the ultrasonic energy, sensors are usually designed in the form of a circular section cut from a thin spherical shell. The energy is emitted from, and received at, the concave surface of the shell. Such a shape has a natural focus at the center of curvature of the spherical shell. In order to maximize performance during reception, the sensor system may be fabricated as an array of small sensors. One widely used design forms a number of annuli from the spherical shell. The return signal at each of the annuli arrives at a slightly different time, and the separate signals can be processed so as to optimize image quality. This type of sensor, called an annular array sensor, is the subject matter of this patent application.
Since ultrasonic energy would be radiated from, and received from, both the concave (desired) side and the convex (undesired) side, the coupling of the convex side must be minimized. This is done by providing an acoustically attenuating layer, an acoustic backing, at the convex side of the array.
In present designs, the acoustic backing also serves as the mechanical structure holding the separate annuli together. The fabrication starts with a shell of piezoelectric material cut from a spherical shell. Individual electrical connectors are attached to the convex surface of the shell at the locations where the annuli will be located. The attenuating acoustic backing is then applied over the convex surface. The acoustic backing must be strong enough to hold the sensor elements together. The acoustic backing also encapsulates the electrical connectors at their point of attachment.
The sensor is then formed into an annular array sensor. The spherical shell is cut into annuli using a set of ganged "hole saws". The cuts are made from the concave surface and are made just deep enough to contact the acoustic backing.
Thus there are two major requirements for an ultrasonic transducer array: the array it must be hermetically sealed so that it can function immersed in liquid, and its concave side must be efficiently impedance matched to the immersion medium, which usually has an acoustic impedance similar to that of water.
As previously described, in the present state of the art, the array is formed by cutting a piezoelectric shell into concentric annuli. The cuts are made right through the shell, all the way from the concave surface to the convex surface using a "hole saw". Thus the array consists of a set of separate concentric annuli, and one central disc.
All these elements must be mounted rigidly together to form an array, a separate wire lead must be connected to the convex side of each element, and a ground lead must be connected to the concave side of all the elements. In addition, the array must be hermetically sealed, since liquid inside the array would disrupt the proper operation of the array. It is further necessary to provide a good impedance matching coating on the concave face of this array.
In the present state of the art, the first coating applied to the concave side of the array must meet three separate requirements:
a. It must be a good electrical conductor. PA0 b. It must form a hermetic seal to the piezoelectric elements. PA0 c. It must have good acoustical impedance characteristics.
These requirements are in conflict with one another; there is no single material which can meet all three requirements well. Graphite is probably the best material known; yet graphite has a number of deficiencies: its impedance is not optimum, it is difficult to hermetically seal the bond between graphite and the piezoelectric material, and it is fragile.
There is a strongly felt need in this industry for an ultrasonic transducer array which can simultaneously provide mechanical integrity, a hermetic seal, and good impedance matching to water, with no compromise of electrical or mechanical performance.